Water pressure

Let me preface this by saying that I am not a professional plumber or well expert. I have completely renovated 2 full bathrooms, and have sweated many pipes, so I have some idea of pluming. On to the issue at hand.
My wife and I have a 100 year old house with a shallow well located 150 feet from the house. The well is 36 feet deep, and I believe it is 10-15 years old. I have replaced the water pump with a like model, a Flotec FP1400 1/2HP pump, but did not replace the impeller. I just replace the pressure switch which I got from a pluming supply store (VAMAC). The pump supplies a water pressure tank, and the water to the house is inbetween the pump and pressure tank. There is a shutoff valve to the house after that intersection. I have a junction that I can unscrew and access the pipe. This is the set up. Now onto the issues.
After replacing the pressure switch, I was getting a lot "sediment" that was rust colored, and would break apart if you rub it in between your fingers. Wife found something on the internet about it being rust or iron bacteria. This sediment was clogging my bathroom spigots. So I took apart the juction in the pump house, and then blew out the pipes with the air compressors from the kitchen sink and bathroom sink, twice. Got a lot of junk out, and the water ran clean after that. Now the problem that we are getting is the water pressure coming out of the sink is fine for about 10-15 seconds, slows down, then goes back to normal pressure. When I go to the pump house and watch the pressure, the pressure starts at 50, then when it gets to about 35, I hear a gurgle in the pressure tank, then there is a quick drop to 32, then the water comes on at 30. The system will hold 50 psi without dropping for at least 15 minutes, both the entire house, and if I shut off the supply line to the house so that it is just the pump house. I can not get the psi to go over 52 psi. I did release just a bit of air out of the schrader valve and did not see any water come out.
My question is, what would possibly cause the water to drop in pressure? I am thinking that I have air in the pressure tank. My plan is to empty the system, and then make sure the pressure is 28 psi when the tank is empty. Any ideas I would be more than happy to hear, and much appreciated!

Well, it has been a long weekend, but I was able to fix the fluctuating water pressure. In my previous comments, the switch was from the supply house VAMAC. It was a Square D pressure switch. I was able to replace the motor with out replacing the impeller by removing the 4 mounting bolts from the old motor and then put the new motor in its place. I didn't replace the impeller because then I would of had to redo the entire pump house pluming, and did not have the parts at the time.
Well, I have now gone from a 20 gallon pressure tank to a 52 gallon pressure tank. I installed a new Flotec 1/2 HP water pump, as it pumped 12 gallons per minute, where every other brand pumped 9. I redesigned the pluming in the pump house, adding a sediment filter and eliminated 8 90 degree elbows. I also replaced a Ruud 60 gallon hot water heater with a GE(American) 80 gallon water heater with the lifetime warrantee. I now have constant pressure with no drops in pressure even with the pump turns on. Lesson learned is replace the impeller when you replace the pump. I decided to change out the pressure tank as it was 11 years old, and with only being 20 gallons, it was just 2 small. Lot of work, but glad I now have an almost entirly new pluming system.

I hate to be the bearer of bad news, but chances are you will be doing this again in the very near future. The pump and tank you bought are not meant to last very long. In my experience about 1.5 years is a normal lifetime.